The protection of metallic or other structures against an underwater environment is a centuries old art. An underwater structure, typically steel, is subjected to electrochemical corrosion and biological fouling, unless protected, typically by coatings. Coatings have been used instead of using more resistent structural materials to minimize cost.
To be effective, a corrosion protection coating must either totally encompass the structure excluding water contact or be preferentially attacked and consumed. Exclusive-type coatings must also withstand the structural loads and erosive forces of the moving water environment in order to maintain water exclusion integrity. A single breach of the exclusive-type coating to completely prevent water from contacting the underwater structure can lead to accelerated corrosion attack and early failure of the steel structure.
Preferential attack-type coatings must be in electrical contact with the structural material to prevent corrosion. This type of protection is based on the different aqueous electrode potentials or galvanic potential of metals and the current flow which occurs when two dissimilar metals are in electrical contact. Because of the electrochemical nature of corrosion, a more active (or anodic) material will be preferentially attacked (corrosion rate increased) when placed in contact with a more cathode material in water. Because of this preferential attack, a galvanic coating of this type will remain effective even if it does not completely exclude contact between water and structure. Protection lasts until the coating is consumed by the accelerated corrosion. Because of this property, galvanic coatings which fully cover the structure offer double protection by first excluding contact with water until breached, then preferentially being consumed.
Protection against fouling by living waterborne organisms, such as barnacles, is also desirable in underwater structures. Organisms add weight and resistance to water flow, they may also cause biological corrosion. Removal of attached organisms may also damage the structure. The most effective anti-fouling materials or coatings tend to be based on metals more noble or cathodic than steel, such as copper, copper-nickel alloys or tin. These materials appear to act as a poison in nearby water as they corrode. Another anti-fouling technique is to enclose the structure with an ablative material. Still another anti-foulng technique is to completely enclose the steel structure with an extremely smooth surface, which makes it difficult for marine organisms to attach themselves. Thus a smooth copper alloy coating offers multipe protections. The alloy composition and thickness can be varied to achieve maximum protection and the coating does not need to completely enclose the structure to be effective against fouling.
The quest to obtain the maximum protection against both corrosion and fouling has struck against the basic inconsistency of using anodic and cathodic materials to protect the structure. Copper in electrical contact with steel will protect against fouling but will accelerate the steel's corrosion. Alternately, an anodic corrosion protection material, such as aluminum will do little to protect against fouling.
Previous efforts to provide multiple coatings tend to rely on aluminum and/or copper base paints. Thick coatings of these paints are not easily controlled and have difficulty withstanding the erosive water forces. They also tend to be attacked and dry quickly in air, requiring immersion relatively soon after application to be effective. They provide a limited shelf life and generally offer only short term protection. Organic sealers over galvanic coatings offer some fouling protection by exposing a smooth surface difficult for organisms to attach to. But fouling once established, quickly spreads, requiring scraping. Inorganic coatings over galvanic layers again offer some fouling protection by exposing an extremely smooth exterior. However, the erosive forces and particles of moving water soon roughen the inorganic coating allowing fouling to proceed. Other problems with previous multiple layer systems include bonding strength, limited shelf life, and delamination loss due to differential thermal expansion.